1. Field of the Invention
The present invention relates to a video amplifier circuit with gain and alignment control, for converting an input signal into an output signal having a mean value equal to a reference voltage and variations which are amplified with respect: to the variations in the input signal with a gain controlled by a gain control signal.
2. Description of the Prior Art
The circuit has a particular application in the shaping of television signals, for example to the D2MAC standard, intended to be digitally processed in order to produce a television image. Such a television signal usually includes reference sequences making it possible to situate the values of the signal which may vary between a minimum level, known as "black level", and a maximum level, known as "white level". In order to optimize the definition of the image, it is desirable for the analog signal to be amplified so that the interval between the minimum and maximum levels substantially covers the full scale of the analog/digital converters which convert the signal into digital data to be processed. This necessitates, on the one hand, amplification with gain controlled as a function of the amplitude of the input signal, this amplitude being capable of varying due to the source of the signal or to its transmission conditions, and, on the other hand, level adjustment intended to align the amplified signal with respect to the thresholds of the analog/digital converters.
In order to align the signal, a locking technique (clamp) is usually used, in which "clamping" technique the signal passes through a capacitor which is charged to a clamping voltage included in an identifiable reference sequence of each line of the input signal. The capacitor is charged during the reference sequence and should keep a constant charge during the rest of the line. This condition is very difficult to fulfill when the reference sequence is very short compared to the duration of a line, which is the case especially for signals to the D2MAC standard, for which a line lasts 64 .mu.s and includes a reference sequence of about 750 ns.
The present inventors have adopted an approach including controlling the mean value of the output signal and the amplification gain. These two quantities are determined by the digital processors which analyze the output signal of the circuit. The mean value to be applied depends on the contents of the image to be presented and more precisely on the proportion of white in the image. The gain to be applied depends on the amplitude of the variations in the input signal between the maximum and minimum levels, which is independent of the contents of the image.
In order to control the mean value of a signal, one technique is known for compensation by a DC loop, in which the signal is periodically sampled in order to produce an error voltage in the event of drift with respect to the reference voltage, a feedback loop acting to adjust an appropriate DC component when a drift is detected. When the alignment controlled by the DC loop takes place before amplification, the reference voltage should depend on the amplification gain and hence on the amplitude of the input signal while the error voltage is independent of the input signal, which is unacceptable. When the controlled-gain amplification is carried out first, followed by the alignment control by the DC loop, the two controls are certainly made independent, but the variation in the mean value of the input signal with respect to the power supply voltage of the circuit gives rise to unacceptable blocking and saturation phenomena.
The object of the present invention is to provide a video amplifier with gain and alignment control, making it possible to control the mean value of the output signal and the amplification gain independently.